In the framework of a monitoring program of quality of care for breast cancer in general hospitals ongoing in Italy since 1983 we have analyzed the epidemiological profile of oestrogen receptor positive (ER+) and oestrogen receptor negative (ER-) breast cancer patients. A total of 248 cases were interviewed: 154 were ER+ and 94 ER- patients. The frequency of ER+ cases increased with age, the trend being, however, of borderline statistical significance. Similarly, ER + breast cancer increased with the number of live births (chi 2(1) for linear trend adjusted by age 0, 1, and greater than or equal to 2 = 6.7; p = 0.01). A negative association emerged between ER+ status and menstrual irregularities (odd ratio adjusted by age = 0.4; 95% confidence interval = 0.2-1.0), while no relation was observed between oestrogen receptor status and menopausal status, age at 1st birth, age at menarche, and oral contraceptive and oestrogen replacement therapy. The present findings suggest that some epidemiological differences exist between ER+ and ER- breast cancer patients; however, large uncertainties emerge in our and previous studies, suggesting the need for further biologic and epidemiologic research to better define the clinical and biological significance of hormonal receptor status.